The EU presidency on Saturday called for the establishment of an international tribunal for war crimes after new mass graves were found in Ukraine.

"In the 21st century, such attacks against the civilian population are unthinkable and abhorrent," said Jan Lipavsky, foreign minister of the Czech Republic which holds the European Union's rotating presidency.

"We must not overlook it. We stand for the punishment of all war criminals," he added in a message on Twitter.

"I call for the speedy establishment of a special international tribunal that will prosecute the crime of aggression."

The appeal follows the discovery by Ukrainian authorities of around 450 graves outside the formerly Russian-occupied city of Izyum with most of the exhumed bodies showing signs of torture.

"Among the bodies that were exhumed today, 99 percent showed signs of violent death," Oleg Synegubov, head of Kharkiv regional administration, said on social media.

"There are several bodies with their hands tied behind their backs, and one person is buried with a rope around his neck," he added.

An AFP journalist at the site saw at least one body with its hand tied with cord.

'Murderers, torturers'

US Secretary of State Antony Blinken said the mass graves likely provided more evidence that Russia is committing war crimes in its pro-Western neighbour. French President Emmanuel Macron described what had happened in Izyum as atrocities.

The Ukrainian parliament's human rights commissioner, Dmytro Lubinets, said there were "probably more than 1,000 Ukrainian citizens tortured and killed in the liberated territories of the Kharkiv region".

Ukraine national police chief Igor Klymenko said they had found multiple torture rooms in the town of Balakliya and elsewhere in Kharkiv since the Russians were driven out.

The United Nations in Geneva has said it hopes to send a team to determine the circumstances of the deaths.

The macabre discoveries came a little more than five months after the Russian army, driven out of Bucha near the capital Kyiv, left behind hundreds of corpses of civilians, many of whom bore the traces of torture and summary executions.

"The world must react to all this," Zelensky said in a video.

"Russia has repeated in Izyum what it did in Bucha. And now we have just begun to learn the full truth about what was happening in Kharkiv region at that time".

'Deeply shocked'

The European Union is "deeply shocked" at the discovery by Ukrainian officials of mass graves in the recaptured city of Izyum, the bloc's foreign policy chief Josep Borrell said Friday.

"This inhuman behaviour by the Russian forces, in total disregard of international humanitarian law and the Geneva conventions, must stop immediately.

On Thursday, EU chief Ursula von der Leyen said she wanted Russian President Vladimir Putin to face the International Criminal Court over war crimes in Ukraine.

In Washington, US President Joe Biden warned his Russian counterpart Vladimir Putin against using chemical or tactical nuclear weapons in the wake of serious losses in his war in Ukraine.

"Don't. Don't. Don't," Biden said, in an excerpt from an interview with CBS' "60 Minutes" aired Friday evening.

"You would change the face of war unlike anything since World War II," Biden said.

'Pushing them back'

On the ground, Ukrainian forces have recaptured thousands of square kilometres in recent weeks thanks to a counter-offensive in the north-east and now threaten enemy positions in the south, as the fighting and bombings continue.

The Russians "are angry because our army is pushing them back in its counter-offensive," said Svitlana Shpuk, a 42-year-old worker in Kryvyi Rih, a southern town, and Zelensky's hometown, which was flooded after a dam was destroyed by Russian missiles.

The governor of the Kharkiv region, Oleg Synegoubov, said an 11-year-old girl had been killed by missile fire in the region.

Pavlo Kyrylenko, the governor of Donestk in eastern Ukraine which has been partially controlled by Russian-backed separatists since 2014, said on social media that a thermal power plant was "shelled by Russian invaders" on Saturday morning in Mykolaivka.

Ukrainian firefighters were battling the blaze, he said, adding that the Russian shelling had led to cuts in drinking water.

"The occupiers are deliberately targeting infrastructure in the area to try to inflict as much damage as possible, primarily on the civilian population," he charged.

He had earlier reported that two civilians had been killed and 11 wounded in the past 24 hours by Russian fire.

Few residents on the streets

In its daily briefing in Moscow, the Kremlin said it had carried out "high-precision" strikes against Ukrainian positions in the Mykolayev and Kharkiv regions.

In the northeastern town of Kupainsk, which was recaptured last week by Ukrainian forces, clashes continued with the Russian army entrenched on the eastern side of the Oskil river.

Few residents ventured out into the streets where Ukrainian soldiers and volunteers were moving about.

A column of smoke rose over the east of the city, where an ammunition depot was burning.

In the center of the small town, the damaged police station stood deserted, the tattered red flag of the Russian army lying on the ground outside.

The Ukrainian army in a statement said "the enemy carried out four missile strikes and 15 air strikes during the day, as well as more than 20 multiple rocket launcher strikes on civilian and military sites in Ukraine".

In the relative calm of Kyiv on Saturday, hundreds of Ukrainians took part in a farewell ceremony at the Kiev national opera house for former ballet dancer and later teacher Oleksandr Shapoval. He was killed at the age of 47 in the east of the country while fighting the Russians.

Shapoval was hit by mortar fire on September 12, near the town of Mayorsk in the Donetsk region.

Meanwhile Ukraine's Zaporizhzhia nuclear plant began receiving power from the national grid once again, the UN's atomic agency (IAEA) said Saturday, after it was cut off from external power, raising the risk of an accident.

The Russian-occupied plant, the largest in Europe, had been cut from the national grid since September due to shelling.